Dampers have long been used in vibration and/or shock attenuating systems, such as the suspension systems of automobiles. Although the dampers may take other forms, they commonly comprise piston and cylinder assemblies having variable volume chambers interconnected by an orifice or passageway through which hydraulic fluid is displaced and by which such fluid is "throttled" to an extent that is a function of the size of the orifice. Most such dampers are of a purely "passive" type that undergo extension and compression, with ensuing generation of damping forces, only in response to relative movement of the spaced members (such as the body and frame components of an automobile) interconnected by the damper. The damping forces of passive dampers always oppose the relative movement and will under certain operational conditions undesirably amplify, rather than attenuate the transmission of vibrations and forces between the members interconnected by the damper.
The foregoing limitation of passive dampers is not shared by fully "active" ones. In addition to a piston and cylinder assembly, active fluid dampers include a pump or similar source of pressurized fluid, together with rapidly-acting control means for regulating the flow of fluid between such source and the assembly. This enables both the magnitude and the direction of the damping forces generated by the damper to be substantially independent of the input or excitation velocity, which in turn may and usually will result in superior vibration attenuation. The foregoing advantage of active dampers over passive ones is offset to a significant extent, however, by their normally being considerably more costly, bulky and/or heavy due to their inclusion of a pump or similar source of high pressure fluid. Any one of the foregoing factors may render active dampers unsuitable for many applications.
There are also dampers of the so-called "semi-active" type. A semi-active damper resembles a passive one in that it does not include a pump or other source of pressurized fluid, and can only generate damping forces in response to and in opposition to relative movement between the members interconnected by it. A semi-active damper resembles an active one in its inclusion of means for controlling, albeit to only a limited extent, the damping forces generated by the damper. More specifically, a semi-active damper can at desired times reduce the magnitude of the damping forces generated during operation by relative movement between the member interconnected by the damper. Such control may be realized by the provision of adjustable valve means in association with the orifice or passageway interconnecting the variable volume chambers of the damper, together with control means for dynamically adjusting the valve during damper operation. Simple semi-active dampers have long been employed for special purposes, such as to prevent their "bottoming-out" during extreme extension and/or compression. While these dampers may be satisfactory for specific limited purposes, the degree of vibration attenuation that they achieve is not significantly different from that realized by passive dampers, and does not approach that realized by active ones.
In contrast to the customary type of semi-active damper, the damper disclosed in U.S. Pat. No. 3,807,678 is adapted to and does achieve a degree of vibration attenuation or isolation approaching that realized by a comparable active damper. A semi-active damper of the type disclosed in such patent ideally should generate damping forces of appreciable magnitude only at those times when the effect thereof would be to attenuate the transmission of vibratory and shock forces to the member that is to be isolated. At all other times the damper ideally would not generate any damping forces. While the foregoing ideal result cannot be completely realized in practice, it can be approached and a resulting high degree of vibration attenuation or isolation can be achieved when the damper is operated in accordance with an appropriate control policy. One such policy disclosed in U.S. Pat. No. 3,807,678 is based upon determination of the instantaneous relative velocity between the supported and supporting members interconnected by the damper, and upon the instantaneous absolute velocity of the supported member. When the product of the aforesaid absolute and relative velocities is greater than zero, its control means causes the damper to produce damping forces of either the full magnitude realizable from the then transpiring relative movement between the supported and supporting members, or of a reduced but still significant magnitude adapted to offset the force then tending to undesirably displace the supported member. When the aforesaid product is less than zero the damping force is caused to be of minimal (ideally zero) magnitude. The aforesaid control policy produces excellent results. However, it is somewhat difficult and costly to implement since the absolute velocity of the supported member cannot readily be directly ascertained, and is at times indirectly calculated as by integration of output data received from an accelerometer associated with the supported member. This relatively expensive way of implementation detracts from the control policy, and may render it unsuitable for some semi-active damper applications.